It's doubtful you can pick one up cheap, it's a specialty item, for which your particular screen has to be compatable with. http://www.sageinc.com is pretty well the only supplier of these cards at appx. $300.00 a pop.. and you have to wire the screen up correctly, and even then it ain't a guarantee that the thing is gonna work, lots of people have thought about doing this, many have tried, and very few have been successful. Not to deride your efforts, if you can do it, all the more power to you... it's just something that is chancy to have success with.
Lots of SBCs (Single Board Computers) have onboard LCD controllers built-in to them. If it's going to cost $300.00 for a controller card, then you can get a Pentium SBC with onboard LCD controller for around $360.00.